Townhouse Humidifier placement

Just bought a Levoit Superior 6000s for our townhouse.

Is it better to put it on the main (2nd) floor or on the top (4th) floor landing? We have a fairly open staircase from 2nd to 4th. 2 Dedicated HVaC for 3/4 floor and 2/1 floor. Thermostats are on 2nd and 4th floor.

4th usually reads around 10-14% higher humidity than 2nd but I think that’s more about the difference in stat models.

My assumption was 4th floor is better as the moisture would naturally “fall” to 2nd floor but I’m wondering if my logic is backwards.

The Levoit can reach the 50% setting but the thermostats only show a 2-4% increase in humidity. I’m wondering if it will take a few days to reach a more stable point.